Study of the acoustoelectric effect for SAW sensors

Abstract

Research has recently begun on the use of ultrathin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., because the basic material parameters may change because of film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, and which also correspond to the mid-point between the open and short-circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on a YZ LiNbO3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, exposure to hydrogen gas, or both. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

SAW Sensors for Chemical Vapors and Gases

“…They observed a large change in the frequency when using the conductivity as the major interaction mechanism whereas almost negligible frequency shift was observed when considering the mass-loading as the major mechanism (Figure 4b). Later, other research groups (e.g., Lec et al [88], Fisher et al [89]) have shown theoretically and experimentally that some films can have much larger effect of conductivity change than of the mass and viscoelasticity changes to the wave propagation upon exposure to gases. The development of the chemical sensors using the mass loading and acoustoelectric effect and their sensing applications will be discussed in later sections.…”

In situ observation and measurement of the SAW thin-film acoustoelectric effect

“…These measurements were performed using standard saW delay lines on a single wafer, from 14 different devices operating at fundamental and harmonic frequencies. The experimental procedure and data extraction were previously published in [5].…”

“…Initial in situ measurements of Ti's sheet resistance as a function of film thickness, R s (t), were performed with the use of a thin-film resistor [3], [5]. a plot of the measured sheet resistance versus thickness and the data fit using (7) is presented in Fig.…”

“…hence, if properly implemented, the saW-thin-film acoustoelectric interaction is useful in the analysis of the properties of thin films, and in thin-film physical sensors [1], [2]. This work is part of a continuing effort to develop a passive, wireless, orthogonal frequency-coded (oFc) saW rFIdtag hydrogen-gas sensor [3]- [5], a schematic of which is presented in Fig. 1.…”

Gas sensing using thermally actuated dual plate resonators and self-sustained oscillators

“…Chemical hydrogen sensors [1][2][3][4][5][6][7][8] for examples, utilize palladium (Pd) [2,6,[8][9] and platinum (Pt) [1,[3][4][5]7,10] as catalyst to absorb hydrogen, inducing volume expansion [7][8][9] of the Pt/Pd layer. Furthermore, the Pd/Pt breaks hydrogen gas into hydrogen ions leading to a chemical reaction between the hydrogen ions and the oxide layer in the sensors changing the optical refractive index [2] or the electrical properties [3][4][5] of the sensing layer.…”